diff --git a/Library/Homebrew/utils/ruby.sh b/Library/Homebrew/utils/ruby.sh
index 7d09e4d83a..fe1e686f01 100644
--- a/Library/Homebrew/utils/ruby.sh
+++ b/Library/Homebrew/utils/ruby.sh
@@ -1,5 +1,7 @@
 export HOMEBREW_REQUIRED_RUBY_VERSION=2.6.3
 
+# HOMEBREW_LIBRARY is from the user environment
+# shellcheck disable=SC2154
 test_ruby() {
   if [[ ! -x $1 ]]
   then
@@ -7,20 +9,23 @@ test_ruby() {
   fi
 
   "$1" --enable-frozen-string-literal --disable=gems,did_you_mean,rubyopt \
-    "$HOMEBREW_LIBRARY/Homebrew/utils/ruby_check_version_script.rb" \
-    "$HOMEBREW_REQUIRED_RUBY_VERSION" 2>/dev/null
+    "${HOMEBREW_LIBRARY}/Homebrew/utils/ruby_check_version_script.rb" \
+    "${HOMEBREW_REQUIRED_RUBY_VERSION}" 2>/dev/null
 }
 
+# HOMEBREW_MACOS is set by brew.sh
+# HOMEBREW_PATH is set by global.rb
+# shellcheck disable=SC2154
 find_ruby() {
-  if [[ -n "$HOMEBREW_MACOS" ]]
+  if [[ -n "${HOMEBREW_MACOS}" ]]
   then
     echo "/System/Library/Frameworks/Ruby.framework/Versions/Current/usr/bin/ruby"
   else
     IFS=$'\n' # Do word splitting on new lines only
-    for ruby_exec in $(which -a ruby 2>/dev/null) $(PATH=$HOMEBREW_PATH which -a ruby 2>/dev/null)
+    for ruby_exec in $(command -v -a ruby 2>/dev/null) $(PATH=${HOMEBREW_PATH} command -v -a ruby 2>/dev/null)
     do
-      if test_ruby "$ruby_exec"; then
-        echo "$ruby_exec"
+      if test_ruby "${ruby_exec}"; then
+        echo "${ruby_exec}"
         break
       fi
     done
@@ -28,14 +33,17 @@ find_ruby() {
   fi
 }
 
+# HOMEBREW_FORCE_VENDOR_RUBY is from the user environment
+# HOMEBREW_MACOS_SYSTEM_RUBY_NEW_ENOUGH are set by brew.sh
+# shellcheck disable=SC2154
 need_vendored_ruby() {
-  if [[ -n "$HOMEBREW_FORCE_VENDOR_RUBY" ]]
+  if [[ -n "${HOMEBREW_FORCE_VENDOR_RUBY}" ]]
   then
     return 0
-  elif [[ -n "$HOMEBREW_MACOS_SYSTEM_RUBY_NEW_ENOUGH" ]]
+  elif [[ -n "${HOMEBREW_MACOS_SYSTEM_RUBY_NEW_ENOUGH}" ]]
   then
     return 1
-  elif [[ -z "$HOMEBREW_MACOS" ]] && test_ruby "$HOMEBREW_RUBY_PATH"
+  elif [[ -z "${HOMEBREW_MACOS}" ]] && test_ruby "${HOMEBREW_RUBY_PATH}"
   then
     return 1
   else
@@ -43,6 +51,8 @@ need_vendored_ruby() {
   fi
 }
 
+# HOMEBREW_LINUX is set by brew.sh
+# shellcheck disable=SC2154
 setup-ruby-path() {
   local vendor_dir
   local vendor_ruby_root
@@ -56,53 +66,53 @@ setup-ruby-path() {
   local upgrade_fail
   local install_fail
 
-  if [[ -n $HOMEBREW_MACOS ]]
+  if [[ -n ${HOMEBREW_MACOS} ]]
   then
     upgrade_fail="Failed to upgrade Homebrew Portable Ruby!"
     install_fail="Failed to install Homebrew Portable Ruby (and your system version is too old)!"
   else
     local advice="
 If there's no Homebrew Portable Ruby available for your processor:
-- install Ruby $HOMEBREW_REQUIRED_RUBY_VERSION with your system package manager (or rbenv/ruby-build)
+- install Ruby ${HOMEBREW_REQUIRED_RUBY_VERSION} with your system package manager (or rbenv/ruby-build)
 - make it first in your PATH
 - try again
 "
-    upgrade_fail="Failed to upgrade Homebrew Portable Ruby!$advice"
-    install_fail="Failed to install Homebrew Portable Ruby and cannot find another Ruby $HOMEBREW_REQUIRED_RUBY_VERSION!$advice"
+    upgrade_fail="Failed to upgrade Homebrew Portable Ruby!${advice}"
+    install_fail="Failed to install Homebrew Portable Ruby and cannot find another Ruby ${HOMEBREW_REQUIRED_RUBY_VERSION}!${advice}"
   fi
 
-  vendor_dir="$HOMEBREW_LIBRARY/Homebrew/vendor"
-  vendor_ruby_root="$vendor_dir/portable-ruby/current"
-  vendor_ruby_path="$vendor_ruby_root/bin/ruby"
-  vendor_ruby_terminfo="$vendor_ruby_root/share/terminfo"
-  vendor_ruby_latest_version=$(<"$vendor_dir/portable-ruby-version")
-  vendor_ruby_current_version=$(readlink "$vendor_ruby_root")
+  vendor_dir="${HOMEBREW_LIBRARY}/Homebrew/vendor"
+  vendor_ruby_root="${vendor_dir}/portable-ruby/current"
+  vendor_ruby_path="${vendor_ruby_root}/bin/ruby"
+  vendor_ruby_terminfo="${vendor_ruby_root}/share/terminfo"
+  vendor_ruby_latest_version=$(<"${vendor_dir}/portable-ruby-version")
+  vendor_ruby_current_version=$(readlink "${vendor_ruby_root}")
 
   unset HOMEBREW_RUBY_PATH
 
-  if [[ "$HOMEBREW_COMMAND" == "vendor-install" ]]
+  if [[ "${HOMEBREW_COMMAND}" == "vendor-install" ]]
   then
     return 0
   fi
 
-  if [[ -x "$vendor_ruby_path" ]]
+  if [[ -x "${vendor_ruby_path}" ]]
   then
-    HOMEBREW_RUBY_PATH="$vendor_ruby_path"
-    TERMINFO_DIRS="$vendor_ruby_terminfo"
-    if [[ $vendor_ruby_current_version != "$vendor_ruby_latest_version" ]]
+    HOMEBREW_RUBY_PATH="${vendor_ruby_path}"
+    TERMINFO_DIRS="${vendor_ruby_terminfo}"
+    if [[ ${vendor_ruby_current_version} != "${vendor_ruby_latest_version}" ]]
     then
-      brew vendor-install ruby || odie "$upgrade_fail"
+      brew vendor-install ruby || odie "${upgrade_fail}"
     fi
   else
     HOMEBREW_RUBY_PATH=$(find_ruby)
     if need_vendored_ruby
     then
-      brew vendor-install ruby || odie "$install_fail"
-      HOMEBREW_RUBY_PATH="$vendor_ruby_path"
-      TERMINFO_DIRS="$vendor_ruby_terminfo"
+      brew vendor-install ruby || odie "${install_fail}"
+      HOMEBREW_RUBY_PATH="${vendor_ruby_path}"
+      TERMINFO_DIRS="${vendor_ruby_terminfo}"
     fi
   fi
 
   export HOMEBREW_RUBY_PATH
-  [[ -n "$HOMEBREW_LINUX" && -n "$TERMINFO_DIRS" ]] && export TERMINFO_DIRS
+  [[ -n "${HOMEBREW_LINUX}" && -n "${TERMINFO_DIRS}" ]] && export TERMINFO_DIRS
 }